The Global Positioning System (GPS) is a satellite-based navigation system developed and operated by the U.S. Department of Defense. GPS permits land, sea, and airborne users to determine their three-dimensional positions, velocities, and time.
GPS uses NAVSTAR (NAVigation Satellite Timing and Ranging) satellites. The current constellation consists of 21 operational satellites and 3 active spares. This constellation provides a GPS receiver with four to twelve useable satellites from which it can receive GPS satellite signals at any time. A minimum of four satellites allows the GPS receiver to compute its GPS position (latitude, longitude, and altitude) and GPS system time. Altitude is typically referenced to mean sea level.
The GPS satellite signal from the NAVSTAR GPS satellites contains information used to identify the satellite, as well as to provide position, timing, ranging data, satellite status, and the updated ephemeris (orbital parameters).
A worldwide system of tracking and monitoring stations measures signals from the GPS satellites and relays information from the signals to a Master Control Station. The Master Control Station uses this information to compute precise orbital models for the entire GPS constellation. This information is then formatted into updated navigation messages for each satellite.
The GPS receivers contain antennas, reception equipment, and processors that are utilized for determining position and timing based on satellite ranging.
Current designs utilize frequency-specific hardware at the front end of GPS receivers to perform signal acquisition and tracking functions. Such a GPS receiver includes a GPS front end and a system processor. The GPS front end receives the incoming analog GPS signal from a GPS antenna. This analog GPS signal is amplified by a low noise amplifier and is filtered by a bandpass filter. The low noise amplifier and the bandpass filter may be provided in multiple stages, if desired.
The analog GPS signal at the output of the bandpass filter is downconverted in a frequency downconverter by mixing the analog GPS signal with a local oscillator signal to shift the carrier frequency of the incoming analog GPS signal to a lower and more manageable frequency band. This downconversion can be performed multiple times to bring the frequency of the analog GPS signal down in steps to the final desired frequency. Each mixing operation produces a high frequency information band along with the desired lower frequency band. Accordingly, each mixing stage of the frequency downconverter requires a bandpass filter to remove the information in the higher frequency band. A frequency reference is provided for the frequency downconverter. As indicated above, the GPS front end is typically implemented in hardware.
The system processor includes an A/D sampler which, in response to the signal from the frequency reference, converts the downconverted analog GPS signal to a digital GPS signal. A GPS processor uses the downconverted digital GPS signal to first determine ranging and satellite information for the GPS satellites in the line of sight of its antenna at the time, and to then determine latitude, longitude, altitude, and/or GPS system time.
The mixing stages and band pass filters provided by the frequency downconverter comprise typical frequency specific hardware in the GPS front end of the GPS receiver. The design and development of this mixing and band pass filter hardware is an expensive part of the GPS receiver. Not only does this mixing and band pass filter hardware add recurring cost, but mixing circuit designs and re-designs add non-recurring cost whenever even a small change is made to the frequency characteristics of the system. Furthermore, this frequency-specific hardware also typically suffers from changes as a result of aging.
In order to lower cost and to reduce the effects of aging hardware, it may be desirable to remove this hardware and replace its function with software. In particular, a software solution where signal acquisition is executed with frequency domain processing and signal tracking is executed with time domain processing has shown to be a promising technique. Frequency domain processing techniques may involve Fast Fourier Transforms (FFT) for Wavelet Multiresolution Analysis (WMA). However, implementation of such a software based GPS receiver is challenging. One particular challenge encountered in designing a software based GPS receiver is in performing the handover between frequency domain acquisition and time domain tracking. This challenge arises because, if software based GPS position acquisition is done in the frequency domain with a “batch” of data, acquisition can take a non-trivial amount of time and, thus, there is a time gap between when the data used for acquisition is collected and when the acquisition solution is actually computed. Moreover, implementing signal acquisition in the frequency domain may be desirable for reasons other than reducing hardware, in which case the time required to hand over processing from acquisition to tracking is still a problem.
The present invention overcomes one or more of these or other problems.